Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes

• Published in: Journal of the American Chemical Society 2020-06, Vol.142 (24), p.10692-10704
• Main Authors: Gransbury, Gemma Kate, Livesay, Brooke N, Janetzki, Jett T, Hay, Moya Anne, Gable, Robert W, Shores, Matthew P, Starikova, Alyona, Boskovic, Colette
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.0c01073

Bis(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step valence tautomeric (VT) interconversions of the form {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII} ⇌ {CoII-SQ-SQ-CoII} (cat2- = catecholate, SQ•- = semiquinonate). The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol (spiroH4) or 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol (Br4spiroH4) with Mentpa ancillary ligands (tpa = tris(2-pyridylmethyl)amine, n = 0‒3 corresponds to methylations of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic and DFT computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step partial VT, two-step VT, incomplete VT, and temperature-invariant {CoIII-cat-cat-CoIII} states are observed. Electrochemistry, DFT calculations and the study of electronic communication in {ZnII-cat-SQ-ZnII} and {ZnII-SQ-SQ-ZnII} analogs have allowed elucidation of thermodynamic parameters governing the one- and two-step VT behavior. Parameters based on 1) the degree of electronic communication in the bis(dioxolene) ligand and, 2) the matching of cobalt and dioxolene redox potentials, provide direct control over two-step switching properties and confer an advantage for dinuclear VT complexes over two-step spin crossover complexes for future applications in devices or materials.